1. Field of the Invention
The present invention relates to video displays. More particularly, the present invention relates to a combined vertical filter for processing computer graphic images.
2. The Background Art
A desirable feature for a multimedia computer system is the ability to display computer graphics on a standard television (TV) screen. However, the format and characteristics of the two systems differ. Some of the differences are listed in Table 1.
Omitted from Table 1, and this disclosure, are details particular to the various formats of TV such as National Television Standards Committee (NTSC), Phase Alternative Line (PAL) and Sequential Couleur Avec Mxc3xa9moire (SECAM). Those of ordinary skill in the art are familiar with conversions between the various TV formats and the present invention is not intended to be limited to use with any particular format.
Computer graphics images are typically noninterlaced. That is, the entire frame is stored, and displayed, line-by-line from top to bottom. The process repeats and the entire frame is re-scanned at a given refresh rate, for example 75 Hz. In contrast, TV formats typically use two interlaced fields to store and display an image. Each of the two frames contains every other line of the image, such as one with the even numbered lines and the other with the odd. The two fields are consecutively scanned, so that the viewer perceives the resulting image as a blend of the two individual fields. The technique of interlacing was originally developed, in the early days of television, to present a pleasing graphic image while allowing a lower data transfer rate than needed to update the entire image with each scan. Although modern computer systems are not constrained by data transfer rates to the same extent as were early TV systems, interlaced display formats for TV are unlikely to disappear soon.
Overscan is also a remnant of early systems that is now firmly embedded in TV formats. Because it was costly to build TV picture tubes that accurately displayed an image along the outside border, overscan was developed to hide the image border. Typically, 10% to 20% of the image is cropped, or not displayed. For example, the NTSC format contains 486 active horizontal lines per frame but only approximately 430 lines are visible on a TV. Overscan compensation, which makes these hidden lines visible, may be needed when displaying computer graphics on a TV. For example, the xe2x80x9cStartxe2x80x9d icon displayed in Microsoft(copyright) Windows(copyright) that typically appears in the lower left corner of the computer monitor may be outside the displayed area of a TV screen without overscan compensation.
Computer graphics often contain images with high vertical frequency. That is, adjacent horizontal lines may have a high contrast, such as a light rectangular window displayed on a dark background. High vertical frequency is less common in xe2x80x9cnatural imagesxe2x80x9d created for display on a TV. High vertical frequency may lead to a xe2x80x9cflickerxe2x80x9d effect when the image is displayed in an interlaced format on a TV. The flicker effect is caused by the location of the horizontal edge shifting slightly as the two interlaced fields, the even lines and the odd lines, are consecutively scanned. The present invention uses a flicker filter to reduce this effect. Most TV displays do not include a flicker filter.
Another issue when displaying computer graphics on a TV is that of the multiple resolutions often used in computer displays and the fixed resolution of a given TV display format. That is, a computer system may be capable of generating, and displaying graphic images in multiple resolution, such as 640xc3x97480, 800xc3x97600, 1024xc3x97768 and 1280xc3x97960. However, a TV display format is typically defined around a single resolution such as the 486 active horizontal lines used by the NTSC format. In order to display a wide range of computer graphic images on a TV, a multimedia computer system would preferably not be limited to any one image resolution.
The present invention is directed at noninterlaced to interlaced conversion, vertical overscan compensation, vertical scaling, filtering high vertical frequency images and accepting computer graphic images in a variety of resolutions for display on a TV. While there are prior art techniques for individually performing these processes, the present invention efficiently combines the processes.
The last line of Table 1 describes two different color space formats that may be used by computer graphics and TV. These formats are well known to those of ordinary skill in the art, as are the formulas for converting between RGB and YUV color spaces. This conversion is not done by the present invention, although, the present invention may be used in conjunction with hardware or software that performs such a conversion. The present invention may be used with computer graphics using either RGB or YUV color spaces.
An apparatus and method for converting computer graphics images into a format suitable for display on a TV. A flicker filter is combined with a vertical scaling filter and/or vertical overscan compensation filter to produce an interlaced image formatted for display on a TV, more efficiently than if the processes occurred sequentially. The apparatus and method are not limited to any particular filter sizes or set of filter coefficient values. The apparatus and method may be used as part of a multimedia computer system.